linux-kernel - Re: kernFS/sysfs: mmap and vm

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Date:   Wed, 27 Sep 2017 09:45:51 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Federico Vaga <federico.vaga@...a.pv.it>
Cc:     Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: kernFS/sysfs: mmap and vm_operations close

On Wed, Sep 27, 2017 at 08:41:12AM +0200, Federico Vaga wrote:
> On Tuesday, 26 September 2017 23:31:29 CEST Greg Kroah-Hartman wrote:
> > On Tue, Sep 26, 2017 at 05:50:55PM +0200, Federico Vaga wrote:
> > > Hello,
> > > 
> > > I'm writing a sysfs binary attribute that makes use of the `mmap`
> > > operation.
> > Eeek, why?  What are you using that for?
> 
> I have a bus (VME) and we are not using the the VME subsystem provided by the 
> kernel (why? Historical reasons, we had our own implementation before the 
> kernel one appeared and we cannot port hundreds of drivers so easily in our 
> environment).

As the vme subsystem has been in the kernel for a long time (8 years?),
I don't have much sympathy for this, sorry :)

> I want to make our VME interface "as PCI as possible", so I want to re-create 
> resources that users can `mmap` to access the device memory.

Have you looked at the UIO interface?

> This abstraction will allow us to write utilities that works on PCI and VME 
> devices without dealing with the peculiarity of each interface. This is 
> particularly useful when you have FPGAs that can run the "same" code on 
> different buses, but it is true as well when you have the same hardware (the 
> same memory map) installed on VME cards or PCI cards.
> 
> That's why a sysfs VME resource seems to me the easiest and clean way to 
> achieve this. I do not want to create yet another layer that hides the 
> differences between the two buses when mmap is so straight forward and easy to 
> implement. Another point is that adding a new layer add complexity in the 
> architecture and for the developers; they have to learn yet another non-
> standard interface that I invent, while the concept of resource is something 
> that everybody know.

Or if you use the VME kernel interface, everything will "just work"...

> > sysfs binary attributes are for dumping binary data that the kernel
> > doesn't touch/parse, through to hardware.  Why use mmap for this?  Do
> > you have a pointer to your code somewhere?
> 
> No pointer :S

Sorry, I can't help out much then.

Best of luck!

greg k-h